A. Technical Field
The present invention relates to a production process for an unsaturated carboxylic acid (e.g. acrylic acid). More particularly, the present invention relates to a process comprising the step of carrying out catalytic gas phase oxidation of an unsaturated aldehyde (e.g. acrolein) with molecular oxygen or a molecular-oxygen-containing gas by using a fixed-bed multitubular reactor which is packed with catalysts, thereby producing an unsaturated carboxylic acid (e.g. acrylic acid).
The present invention also relates to a production process for an unsaturated aldehyde and/or an unsaturated carboxylic acid. More particularly, the present invention relates to a process comprising the step of carrying out catalytic gas phase oxidation of at least one compound selected from the group consisting of propylene, isobutylene, t-butyl alcohol, and methyl t-butyl ether as a raw material with molecular oxygen or a molecular-oxygen-containing gas by using a fixed-bed multitubular reactor which is packed with catalysts, thereby producing an unsaturated aldehyde and/or an unsaturated carboxylic acid.
B. Background Art
In the case of carrying out catalytic gas phase oxidation of an unsaturated aldehyde (e.g. acrolein) as a raw material with molecular oxygen or a molecular-oxygen-containing gas by using a fixed-bed multitubular reactor which is packed with catalysts, thereby producing an unsaturated carboxylic acid (e.g. acrylic acid) that correspond to each raw material, or in the case of carrying out catalytic gas phase oxidation of at least one compound selected from the group consisting of propylene, isobutylene, t-butyl alcohol, and methyl t-butyl ether as a raw material with molecular oxygen or a molecular-oxygen-containing gas by using a fixed-bed multitubular reactor which is packed with a catalyst, thereby producing an unsaturated aldehyde and/or an unsaturated carboxylic acid that correspond to each raw material, these catalytic gas phase oxidation reactions are accompanied with an extremely exothermic reaction, and therefore a local portion having an extraordinarily high temperature (which may hereinafter be referred to as a hot spot portion) occurs in a catalyst layer.
When the hot spot portion has a high temperature, the hot spot portion excessively causes oxidation reaction to result in lowering a yield, and an excursion reaction is caused if the worst comes to the worst. A catalyst as located at the hot spot portion is exposed to the high temperature, and therefore there is accelerated the deterioration of the catalyst, such as changes of physical properties and chemical properties of the catalyst to result in lowering its activity and the selectivity of the objective product. Particularly, in the case of a molybdenum-containing catalyst (such as molybdenum-bismuth-iron-containing catalysts or molybdenum-vanadium-containing catalysts, hereinafter the same), the composition and properties of the catalyst tend to change due to sublimation of the molybdenum component, and therefore the deterioration extent of the catalyst is large.
The above problems are more striking in the case of carrying out the reaction at a high space velocity or in a high concentration of the raw gas for the purpose of enhancing the productivity of the objective product.
The above problems are explained again. If attention is directed to the entirety of the catalyst layer as packed in the reaction tube, then the catalyst as located at the hot spot portion is more rapidly deteriorated than a catalyst as located at the other portions, and the yield of the objective product is greatly lowered due to longtime use, so its production can be difficult to stably carry out.
In order to cope with such problems, several proposals have hitherto been reported. Examples thereof include: a method that involves diluting a raw-gas-inlet-side catalyst with an inert substance (e.g. JP-B-030688/1978); as a method using a supported catalyst, a method which involves packing a reaction tube with the catalyst in such a manner that the ratio of the catalytic active substance as supported on the support increases from the raw-gas-inlet side toward the raw-gas-outlet side (e.g. JP-A-010802/1995), a method which involves packing a reaction tube with at least two catalysts different as to activity in such a manner that the activity increases from the raw-gas-inlet side toward the raw-gas-outlet side, wherein the catalysts are prepared by changing the kind and/or amount of an alkaline metal as added to the catalysts (e.g. JP-A-336060/2000), a method that involves packing a reaction tube with catalysts in such a manner that the volume of the catalyst as packed in the reaction tube decreases from the raw-gas-inlet side toward the raw-gas-outlet side (e.g. JP-A-241209/1997), and methods as disclosed in such as JP-B-038331/1988, JP-A-294238/1991, JP-A-294239/1991, JP-A-217932/1992, JP-A-003093/1996, and JP-A-168003/1998.
However, in any of these proposals, some extent of improvement is achieved in view of suppressing the temperature of the hot spot portion, but it cannot necessarily be said that the improvement is satisfied in view of the lifetime of the catalyst and the yield of the objective product, and the further improvement is requested under the existing conditions. These problems are remarkable particularly when the reaction is carried out under high-loading conditions (e.g. in a high concentration of the raw gas or at a high space velocity) in the case of using the molybdenum-containing catalyst.
In addition, there is also a problem such that the conventional proposals cannot cope with a method under reaction conditions such that the hot spot portion is formed toward a gas-outlet portion.